Using Docker rootless: how to connect to mail server running on another user

[daks]

Hi,
I'm trying to setup Roundcube with the provided docker-compose files, I'm testing with the fpm-postgres-nginx one.
I'm running it with Docker rootless with a user named 'roundcube'.
I also have my imap/smtp server running on Docker rootless with a different user named 'mail'.

Roundcube runs OK, I see the login page but can't connect, it seems that roundcube can't access the imap server. I see a log for roundcube but none on the imap side.

roundcubemail     | errors: <xx> IMAP Error: Login failed for [email protected] against imap.example.com from 172.128.0.1. Could not connect to ssl://imap.example.com:993: Connection refused in /var/www/html/program/lib/Roundcube/rcube_imap.php on line 211 (POST /?_task=logi
n&_action=login)                             

Is it possible and what sould I do to make it work?
I would prefer to keep both stacks (roundcube and mail server) isolated with two different users.

[pabzm]

It should be possible, and Roundcube doesn't share any state or data backend with the mailserver, so different users shouldn't do any difference.

From the error message I'd look into the logs of the mailserver next.

(I'm not sure if our container images are ready for rootless, but if it works I'd be happy to include a new compose-file as an example!)

[pabzm]

Sorry, I wrote faster than I read.

Are you sure the mailserver container is running? No output at all from it sounds like a fundamental problem.

If it is running properly, try if you can reach the mailserver container from the roundcube container:

docker compose exec roundcube curl -v -u user:password imaps://mailhost/INBOX

[daks]

It may be possible that rootless docker have networking restrictions that rootful don't have. In which case my setup wouldn't work at all :(

I'm still looking for answers about that

[daks]

After searching for hours or days, I finally found a solution to enable a container running in Docker rootless to access the host itself. I will document it here in case it may help someone else:

• use a recent version of Docker. I think it's at least version 26. For me it meant not use Debian packages but instead docker-ce ones from Docker apt repository
• install Docker rootless per user as usual
• configure rootlesskit to enable host loopback access by adding a file ~/.config/systemd/user/docker.service.d/override.conf with

[Service]
Environment="DOCKERD_ROOTLESS_ROOTLESSKIT_DISABLE_HOST_LOOPBACK=false"

• daemon-reload systemd and restart docker service
• enjoy :)

Relevant PR in moby project moby/moby#47352